The present invention relates to optical disk devices for optically reproducing signals from optical disks with an optical pickup, and more particularly to a focus servo activating circuit for initiating a focusing operation by activating a focus servo loop of the optical pickup.
Generally with optical disk devices, a focus servo loop is formed for focusing the optical pickup based on a focus error signal prepared from the output signal of the optical pickup. When signals are to be reproduced, a focus actuator is driven with the focus servo loop held out of operation first to alter the focus position of the pickup in one direction to obtain focus error signal, i.e., an S-shaped focus error signal. FIG. 4(a) shows the S-shaped curve of the focus error signal (hereinafter referred to as the xe2x80x9cFE signalxe2x80x9d), and FIG. 4(b) shows a drive signal for the focus actuator (focus drive signal). The zero-cross point of the FE signal shown in FIG. 4(a) is detected to activate the focus servo loop at the zero-cross point, whereby a focus servo is completed.
In order to avoid the influence of noise or offset that becomes incorporated into the FE signal in detecting the zero-cross point of the FE signal, two fixed threshold values, i.e., first and second threshold values TH1, TH2, are set as shown in FIG. 5. The S-shaped FE signal first passes the first threshold value TH1 in the course of an increase, and thereafter passes the second threshold value TH2 during a decrease, whereupon the focus servo is started up (activated).
Since the magnitude of the FE signal differs, for example, with the reflectivity of the optical disk and variations in the sensitivity of the optical pickup, it is practice to amplify the FE signal for the normalization of the amplitude of the FE signal to obtain an FE signal of definite level. FIG. 6 shows an FE signal as amplified to a normalized amplitude (peak-peak).
However, the amplifier for amplifying the FE signal is limited in the adjusting range of the gain, so that in the case of optical disks, such as CD-RW, which are extremely low in reflectivity, it is not always possible to amplify the amplitude of the FE signal to a normalized amplitude within the gain adjusting range. In such a case, the FE signal is unable to exceed the threshold value TH1 as seen in FIG. 7, giving rise to the problem that the focus servo can not be started up normally.
Accordingly, an object of the present invention is to provide a focus servo activating circuit for optical disk devices which is capable of starting up a focus servo even for optical disks of extremely low reflectivity.
In an optical disk device embodying the present invention, a focus servo activating circuit comprises:
amplification means for amplifying an FE signal within a predetermined gain adjusting range in order to obtain a prescribed amplitude value,
calculation means for calculating one or a plurality of threshold values by multiplying the amplitude of the FE signal amplified by the amplification means by one or a plurality of predetermined coefficients, and
control means for starting up a focus servo upon the amplified FE signal passing the threshold value or values.
With the focus servo activating circuit of the present invention described, the magnitude of one or a plurality of threshold values is variably determined in conformity with the magnitude of amplitude of the FE signal as amplified. For example if the amplitude of the FE signal is smaller than the prescribed value, the value of one or the plurality of threshold values is set to a small value, so that the amplified FE signal is capable of passing the threshold values, whereupon the focus servo is started up.
Stated specifically, the amplification means decreases the gain when the amplitude of the FE signal is in excess of the prescribed value while increasing the gain when the amplitude of the FE signal is below the prescribed value, and the FE signal obtained at the gain thus set is amplified to the prescribed value, or upon the gain thus set reaching a limit of the gain adjusting range, the amplitude of the resulting FE signal is given to the calculation means. With the specific construction, the gain is adjusted within the gain adjusting range so as to obtain the prescribed amplitude value. When the prescribed amplitude value can be obtained within the gain adjusting range, the prescribed amplitude is obtained, whereas if it is impossible to obtain the prescribed value within the adjusting range, an amplitude which is close to the prescribed value to the greatest possible extent can be obtained.
Further stated specifically, the calculation means multiplies the amplitude of the amplified FE signal by two predetermined coefficients to calculate a first threshold value for the FE signal to pass while increasing, and a second threshold value for the FE signal to pass while decreasing to approach a zero-cross point after increasing. With the specific construction, the S-shaped curve is detected by the FE signal passing the first threshold value first, and the zero-cross point of the S-shaped curve is detected upon the FE signal subsequently passing the second threshold value.
With the focus servo activating circuit of the present invention for use in the optical disk device embodying the invention, appropriate threshold values are determined in conformity with the amplitude of the FE signal obtained from an optical disk even if the disk is extremely low in reflectivity. The zero-cross point can therefore be detected reliably, so that the focus servo can always be started up normally.